This disclosure is related to a flatness tester for optical components. In particular, this disclosure is related to determining deviations from flatness of brightness enhancing display films that are used in display devices such as television screens or computer screens.
Brightness enhancing display films generally termed “prism sheets” are used in liquid crystalline display devices in order to collimate the light passing through a liquid crystal display. The brightness enhancing display film generally comprises a series of prisms disposed upon a film substrate. FIG. 1 is an exemplary depiction of the series of prisms disposed upon a film substrate. Each brightness enhancing display film comprises a prism surface and a back surface. The prism surface of the brightness enhancing display film is the surface that comprises the upper surfaces (the air contacting surfaces) of the prisms. The back surface of the brightness enhancing display film is the surface opposed to the prism surface. The back surface is generally flat and is parallel to the base of the prisms that are disposed upon the film substrate as shown in the FIG. 1.
Brightness enhancing display films are manufactured by pressing a malleable material against a prism-shaped mold. The malleable material is disposed upon an optically transparent film substrate. Possible manufacturing processes include melt calendaring, embossing, injection molding, compression molding, casting and curing of thermally cured resin onto a substrate, and casting and curing of UV cured resin onto the film substrate.
It is desirable for the brightness enhancing display film to collimate most of the light incident upon the back surface of the film substrate in the on-axis direction. As can be seen in the FIG. 1, the on-axis brightness is the brightness measured in a direction perpendicular to the back surface of the brightness enhancing display film.
During the manufacturing, storage or transportation of a brightness enhancing display film it is possible for the prism surface or the back surface of the film substrate to become distorted or curved. This distortion or curvature is generally undesirable. If the surfaces of the brightness enhancing display film are curved, the brightness in the on-axis direction can be decreased. In addition, variations in flatness introduced from processing errors to roll imperfections (the brightness enhancing display films are stored in rolls) cause interference rings when viewed in transmission. It is therefore desirable to have a method that can be used to determine the amount of deviation from flatness in the surfaces of the brightness enhancing display film.